arm: Use arm_cmpdi_unsigned for thumb2 as well

arm: Use arm_cmpdi_unsigned for thumb2 as well

This changes code generation from "eors; eors; orrs" (which ranges
from 6 to 12 bytes and requires three scratch registers), to
"cmp; it; cmp" (which is always 6 bytes for register inputs and
requires no scratch registers).

        * config/arm/arm.md (*arm_cmpdi_unsigned): Enable for thumb2.
        * config/arm/arm.c (arm_select_cc_mode): Use it.

From-SVN: r182561